The invention relates to a method of making a press felt, the method comprising forming a base fabric, which comprises at least two super-imposed components, the first of which is a web-side surface component and the second is a base component; weaving the base component of warp and weft yarns and providing a first and a second transverse jointing edges of the base component with seam loops of the weft yarns for interconnection; needling at least one batt fibre layer at least on the base fabric surface facing a web to be dried; and assembling the press felt into a closed-loop piece by arranging the jointing edges of the base fabric edge on edge, whereby the jointing edges form a seam area, where the seam loops intermesh and form a seam loop channel in the transverse direction of the press felt, and a seam yarn, which interconnects the jointing edges, is inserted in the seam loop channel.
The invention also relates to a press felt, which comprises a base fabric and a batt fibre layer attached onto at least the surface of the base fabric facing the web to be dried, and in which press felt the base fabric comprises at least two superimposed components, the first of which is a surface component facing the web to be dried and the second is a base component, and in which press felt the base component is woven of warp yarns and weft yarns, and seam loops of weft yarns are provided at a first and a second transverse jointing edges of the base fabric for interconnection, and in which press felt the press felt can be assembled into a closed loop by arranging the jointing edges of the base fabric edge on edge, whereby the jointing edges form a seam area, where the seam loops intermesh and form a seam loop channel in the transverse direction of the press felt, in which channel the seam yarn inter-linking the jointing edges can be inserted.
The invention further relates to a press felt base fabric, wherein the base fabric comprises at least two superimposed components, the first of which is a surface component facing the web to be dried and the second is a base component; the base component is woven of warp yarns and weft yarns, and seam loops of weft yarns are provided at a first and a second transverse jointing edges of the base fabric for interconnection.
The aim of a press section in a paper machine is to drain a web as efficiently as possible without causing any deterioration in the quality of the web, however. From the viewpoint of energy consumption, it is most advantageous to remove as much water as possible already at the press section, whereby there is less need to dry the paper web at subsequent sections of the paper machine. Depending on the press structure, the press section of the paper machine employs a press felt on one or both sides of the web to be dried, into which press felt the water in the web is absorbed. The purpose of the press felt is to transport the water away after pressing without allowing it to migrate back into the web. In pressing, the paper web is conveyed on the felt to a gap between two rolls, i.e. so-called nip. Typically, there are one to four nips in succession, and in each nip water is pressed out of the web into the felt. Consequently, the felt should be such that the water can well be absorbed into the felt pressed in the roll nip. The press felts comprise a base fabric that provides the felt with a necessary space for water, for instance. To make the felt surface smooth, batt fibre is needled onto at least a base fabric surface facing the paper web. The batt fibre thus prevents markings from being produced on the web to be drained. In addition, by means of the batt fibre the water retention capacity of the felt can be adjusted to a desired level such that water is prevented from migrating back from the felt into the web, i.e. so-called rewetting is prevented.
Several alternative ways to weave a base fabric are known. It is possible to weave the base fabric on a weaving machine directly into a seamless, endless loop. On the other hand, it is possible to form base fabric modules provided with seam loops, and a desired number of base fabric modules can be interconnected to form a closed-loop base fabric. In that case the seam loops at the jointing edges of the base fabric modules are intermeshed and connected by inserting a seam yarn in a channel formed by the seam loops. Typically, the seam loops are formed during weaving. The base fabric can be woven in a flat or horseshoe form in known manners.
The press felts provided with seam loops are generally considered to have an advantage that it is easier, quicker and safer to mount them on a paper machine as compared with the closed-loop press felts. The structure of the press section can also be simpler and less expensive. Even though the press felts provided with seam loops have the above-described good properties, they have certain drawbacks, however, which make the felts with seams less desirable. One drawback is, for instance, that the seam loops form a discontinuity in the felt, which produces marking on the web to be dried in the press roll nip. One reason for this is that the thickness of the base fabric is different at the seam. In addition, because yarn density of the base fabric is different in the seam area, permeance of a finished press felt at the seam is different from the rest of the felt. Because the number of yarns in the seam area is lower, adherence of the batt fibre also poses a problem. Further, during use the felt is subjected to tensile stress in the running direction of the machine, which tends to open up the seam area. Discontinuity of this kind produces marking on the web to be dried. In addition, permeance in the seam area is different from the rest of the structure.
The object of the present invention is to provide a novel and improved press felt provided with seam loops and a method of making the same.
The method of the invention is characterized by extending a surface component at a first jointing edge of a base fabric for a predetermined distance longer than a base component in the longitudinal direction of the press felt, whereby a seam flap extending over at least the seam area of the base component is provided at the first jointing edge when the base fabric is formed; extending the base component at a second jointing edge of the base fabric for the length of the seam flap longer than the surface component; and needling a batt fibre layer on the base fabric provided with the seam flap.
The press felt of the invention is characterized in that, observed in the longitudinal direction of the press felt, the surface component extends longer than the base component at the first jointing edge of the base fabric and forms at the first jointing edge a seam flap that is designed to extend at least over the base component seam area; that at the second jointing edge of the base fabric the base component extends for the length of the seam flap longer than the surface component; and that the batt fibre layer is attached after forming the seam flap.
The base fabric of the invention is characterized in that, observed in the longitudinal direction, the surface component extends longer than the base component at the first jointing edge of the base fabric and forms at the first jointing edge a seam flap that is designed to extend at least over the base component seam area; and that at the second jointing edge of the base fabric the base component extends for the length of the seam flap longer than the surface component.
The basic idea of the invention is that the base fabric of the press felt comprises at least two superimposed components. On the side facing the web to be dried there is a surface component and below the surface component there is a base component. The transverse jointing edges of the base component are provided with seam loops of weft yarns. Observed in the longitudinal direction, the surface component of the base fabric extends over the seam area and forms a protective seam flap that reduces marking on the web. In addition, the seam flap extending over the seam forms a continuous weave layer, onto which the press felt surface batt fibre can be properly attached also in the seam area. It is possible to make permeance in the seam area correspond, at least approximately, to that in the rest of the press felt. Further, the seam flap protects the seam area of the basic layer from wearing. Discontinuity in the structure is minimized by the seams of the surface component and the base component being at different points in the longitudinal direction of the felt.
The basic idea of an embodiment of the invention is that the surface component and the base component of the base fabric are independent base fabric parts woven in the same step. The surface component and the base component have warp and weft yarns of their own. The components are bound together during weaving by arranging a necessary number of surface component yarns to pass through the base component yarns, or vice versa. Alternatively, the surface component is bound to the base component by means of extra warp- and weft-direction binding yarns that are independent of the component structure. At the first jointing edge of the base fabric, the surface component is woven longer than the base component, whereby a seam flap is formed at the first jointing edge. At the second jointing edge, the weaving of the surface component is stopped at a distance corresponding the seam flap length from the base component edge. This structure is quick to manufacture and stable in use.
The basic idea of an embodiment of the invention is that the surface component and the base component of the base fabric are independent, planar portions that are woven in separate steps and superimposed such that at the first jointing edge the surface component extends longer than the base component, and at the second jointing edge, the base component, in turn, extends for a corresponding distance longer than the surface component.
The basic idea of an embodiment of the invention is that the components are woven simultaneously and that the surface component consists of weft yarns that interlace with the warp yarns of the base component. At the first jointing edge of the base fabric the surface weft yarns extend for the length of the seam flap longer than the base component. At the second jointing edge the situation is opposite, i.e.. the base component extends for the length of the seam flap longer than the surface weft yarns. During weaving, extra warp yarns are applied at the first jointing edge, which warp yarns are arranged to interlace with the surface weft yarns such that the seam flap is provided at the first jointing edge of the base fabric. The weave in the seam flap can be different from the rest of the surface layer. In addition, the cross section, dimensions and material of the seam flap warp yarns can be selected independently of the structure of the base fabric. Thus, the structure of the seam flap can have the desired properties. Permeance of the seam flap can be rendered low, whereby the seam flap compensates for the loose weave of the base component in the seam area.